Sunday, May 6, 2018

Always for the Fun and for some tests with the Satellites, I continued the construction of my antennas for the 1296 Mhz with the construction of some Yagis 35 elements. I did not want the elements to cross or be in contact with the boom, I chose the isolated version.
And 35 elements only on a boom of about 3m gives a gain of about 17.8 dBd. Sufficient for me between the ratio of the gain and the length of the boom.
I was inspired by the model and design of F9FT 23cm / 35el., But I built the dipole is symmetrical T rather than a Loop trombone.
To install the distant elements of the boom, I made some tests with supports like Y nylon columns and square support used to fix the ties wraps but it was inconclusive.
Having nothing else, I looked on the Web, supports already built for this kind of application at I0JXX.

Friday, April 20, 2018

The procedure is easy to take all elements and mount them at 90 degree angle on the same boom.
Traditionally circular polarization is generated by feeding two crossed dipoles 90 degrees out off phase. But you can also shift the dipoles 90 degrees in space and feeding then in-phase with the same effect. The advantage is that the two cables in the splitter have equal length and therefore gives identical transformation of antenna impedance
I prefer the mechanical method despite the electrical method (with different length of cable to obtain the 90 deg phase angle). So, I have shifted each V element exactly at ¼ Lambda forward on the boom. So the entire vertical antenna is shifted forward with reference to the horizontal antenna.
The forward shift (Offset) between the two dipoles is 517 mm to obtain 90 deg Phase angle.

Like this the two antennas are excited with a 90 degr. of phase shift. With this now a rotating field is radiated.
By shifting one section of the yagi forward by a quater wavelength the phasing cable to the two antennas becomes equal, impedances becomes equal and the antennas must share the power equally - regardless of the VSWR.
The two antennas must be feed with a splitter. An air line gives by the calculation an impedance of 35,35 ohms : 𝑍𝜆4⁄∗1√𝜀𝑟 =√𝑍𝑝∗𝑍0∗ 1√1 =√25∗50 ∗1= √1250=35,35𝛺
But mechanical splitter is too big at this frequency.
So the splitter can be made from two 75 ohm ¼ lambda equal length of coax cable.

On 144 MHz a quater wavelength coax is 1/4 x 300/144 x 0,66 = 340 mm (the real velocity factor of the coax used must be applied . In this case + 0.66, solid polyethylene dielectric). I use three quater wavelength 75 ohm cable, which gives the same transformation. The cable length of 102 cm allows the point where the three coax ends are joined together far from the dipoles.
Software: For Yagi antenna, I use the VK5DJ John Drew software ‘’Yagi calculator’’ of http://www.vk5dj.com/yagi.html

Sunday, April 15, 2018

After my first success with this kind of antenna in 1992 in VE2 land (antenna homemade helix 9 turns) see pic at Blanville, Quebec of my antenna set up, I decided to build another one with 14 turns.

VE2EK Antenna's set-up at Blanville (Quebec)

Wanting to further improve my Satellites antennas after the construction of the portable 2m / 70cm yagi cross, I set about building a 14-turn "Home-made" Helical Antenna for 432/435MHz.

This 50 ohm impedance antenna allows, when fed with 25/50W of SSB RF, to join the 'Molniya' orbiting satellites like AO-40 and AO-10 very easily!
The gain of this antenna is estimated at 16.5 dBd theoretical.
It's all about giving ideas on how to build a sustainable helicoidal antenna, rather than giving you a recipe to follow.

Note that for the construction of this antenna I chose round metric values;
The diameter of the wire used for the turns and according to ARRL "Antenne Book" should be between 0.006 and 0.05 Lambda.
The following materials were used to build the 14-turn helical antenna:
• Aluminum: 25mm square support, arms reflector supports,
• Teflon PTFE: Spacers
• Galvanized iron: Reflector grid, Bee hive origin !!
• Aluminum: Helical wire (6.35mm (1/4 "dia.) Plumbing pipe
• Stainless steel: All hardware, support.

Notes on the assembly:
• The central support is drilled every 1/2 turns to receive the Teflon PTFE spacers,
• The spacers are drilled for fixing the helical tube at one end. They are also drilled at the other end to be fixed inside the central support.
• Pieces of galvanized wire. are used to fix the turns to the central support in the spacers.
• I painted all steel surfaces with a polyurethane-silicone alkyd paint.
• The coaxial cable used on the antenna is: 10DFB up to the preamplifier. After the preamplifier, Andrew Heliax ½ cable.
• Connector used: Type N, female, panel mount.
• The reflector is designed so that it can be slid on the central support, in order to adjust the TOS. Two screws hold it in place once adjusted.
The final adjustment of the TOS is done by sliding the reflector forwards or backwards. The center pin of connector N, is fixed on the last turn which has a spacing between the back panel ranging from 13mm to 4 mm at the feed point.
**** this antenna can be fixed to the back of the panel with a counter weight or at its equilibrium point on its boom to an isolated support not to disturb the radiation.